Author Topic: Does wrong input of QNH give wrong altitude to Air Traffic Controller?  (Read 622 times)

Offline Stefanie

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Dear Guys,

I understand that wrong input of QNH will give a wrong altitude indication in the cockpit.
As such, will the Air Traffic Controller also received the wrong altitude on their radar screen as well?
If yes, does this apply to aircraft on Mode C, Mode S and ABS-B?
Thank you for the clarification.

Best regards,
Stefanie



Offline Flyingnut

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Does wrong input of QNH give wrong altitude to Air Traffic Controller?
« Reply #1 on: June 04, 2017, 07:16:45 PM »
If I remember correctly from my USAF ATC Radar Maintenance days, 30+ years ago, the transponder send the altitude based on 29.92"/1013mb.  Then it is corrected at the ATC facility.
Marty
PPL - Instrument Rating
8 NW of KORD

Offline Stefanie

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Hi Flyingnut,

I have understand, thank you for the explanation =)

Best regards,
Stefanie

Offline martyj19

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As pointed out, the Mode C altitude sent by the aircraft is the pressure altitude, or the value that the altimeter would read if it were set to standard pressure.  To the extent that an incorrect altimeter setting causes you to fly at an incorrect altitude, then yes, the controller will see that you are not at the altitude you were expected to fly and if this is sufficiently far off, the controller will reissue the altimeter setting and ask you to check your altitude.

Offline Stefanie

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Hi Martyj19,

Thanks for the detail explanation =)

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